/etc/hosts on Mac -> [IP of Debian] mydebian
Public key of Mac has already been put to authorized_key in root@mydebian:/root/.ssh
'UseDNS no' has been put to root@mydebian:/etc/ssh/sshd_config and then SSH server restarted

ssh root@mydebian -> wait for a long time to login
ssh -v root@mydebian -> no delay at all